Recording/reproducing devices for an information recording medium (such as an optical disc) which stores large amount of data have conventionally adopted an art which records information by focusing a laser beam onto an information recording medium and heating the information recording medium, so as to change physical characteristics of the medium. According to the art, binary digital data is recorded on the information recording medium. The binary digital data is expressed by (i) two states including a state where a recording mark is formed on the information recording medium (hereafter, referred to as “mark”) and a state where a recording mark is not formed (hereafter, referred to as “space”) and (ii) each length of the two states. The digital data is called “recording information” herein. Actually, the recording information is recorded on the information recording medium such that: (1) a recording parameter is set in accordance with the recording information to be recorded; (2) the recording parameter changes the laser beam; (3) the laser beam thus changed forms a recording mark on the information recording medium, thereby recording the recording information on the information recording medium.
In recent years, in response to an increase in the amount of data which is processed, recording speed is being increased for the purpose of reducing the processing time of a recording/reproducing device. Even when an information recording medium becomes capable of recording at a high speed, the information recording medium needs to be capable of recording also at a low speed so as to ensure backward compatibility. Therefore, the information recording medium capable of recording at a high speed has a property which is apt to accumulate heat so as to improve its recording sensitivity. The heat accumulation can distort a recording mark, thereby leading to deterioration in a reproduced signal quality. Therefore, in an information recording medium capable of recording at a high speed, it is important to control heat for forming a recording mark.
As an art which is used for controlling heat for forming a recording mark, there has been known write strategy, which is a high-level laser output control technology. The write strategy carries out laser output control in accordance with the recording parameter so as to control heat for forming a recording mark. As the number of recording parameters increases, the control of heat for forming a recording mark becomes complicated. There is a method where a recording parameter is once stored in an information recording medium in a form of a look-up table, and the recording parameter is read out from the look-up table by a recording/reproducing device so as to set the recoding parameter. In this method, the increased number of recording parameters prolongs read-out time of information from the look-up table by using the recording/reproducing device. This causes delay in the timing of starting a recording process. Therefore, it is preferable to use as small number of recording parameters as possible.
In view of this, for example, Patent Document 1: Japanese Unexamined Patent Application Publication, No. 2005-92942 (published on Apr. 7, 2005) discloses an optical disc recording method including a look-up table which defines recording parameters. In the method, the procedure of controlling heat for forming a recording mark proceeds while the table is looked up.
Specifically, Patent Document 1 describes as follows. A recording parameter for a top pulse which controls heat of a front edge, at which a forming process of a recording mark starts, uses a look-up table in which (i) front space lengths of recording marks are classified into four groups: 2T, 3T, 4T, and 5T or longer and (ii) recording mark lengths are classified into three groups: 2T, 3T, and 4T or longer (see FIG. 17). Also, a recording parameter for a last pulse which controls heat of a rear edge, at which a forming process of a recording mark ends, uses a look-up table in which (i) rear space lengths of recording marks are classified into four groups: 2T, 3T, 4T, and 5T or longer and (ii) recording mark lengths are classified into two groups: 3T and 4T or longer (see FIG. 18). That is, in the look-up table disclosed in Patent Document 1, mark lengths of 4T or longer are classified into the same group, and front and rear space lengths of 5T or longer are classified into the same group. This reduces the number of recording parameters. Thereby, heat for forming a recording mark is controlled at a front edge and a rear edge. The symbol “T” herein represents a time equivalent to one cycle of a clock. For example, a mark length of 2T indicates a region (i.e., a recording region) where a recording mark “1” is formed for a time equivalent to two cycles of the clock. Similarly, a space length of 2T indicates a region where no recording mark is formed for a time equivalent to two cycles of the clock.
With a further increase in a recording speed expected in the future, a recording mark will be formed on an information recording medium having improved recording sensitivity. When a recording mark is formed by using a laser beam, heat distribution is high at a rear edge of a recording mark, at which rear edge the forming process of the recording mark ends. Therefore, the increase in the recording speed will strengthen the tendency to accumulate heat at the rear edge. This leads to an increasing necessity for controlling heat at the rear edge more strictly by using the recording parameter.
Patent Document 1, however, uses the same setting for (i) the classification of recording parameters for controlling heat of a front edge, at which a forming process of a recording mark starts and (ii) the classification of recording parameters for controlling heat of a rear edge, which needs to be controlled strictly. Specifically, both types of the recording parameters use a look-up table in which recording mark lengths of 4T or longer are classified into the same group. In addition, Patent Document 1 does not specifically disclose on what basis the recording mark lengths, the front space lengths, and the rear space lengths are classified in the look-up table, which defines the recording parameters, for the purpose of reducing the number of recording parameters. Therefore, although the number of the recording parameters is reduced, there is a problem that a good reproduced signal quality is not guaranteed (i.e., a reproduced signal quality as desired is not guaranteed) in reading out a recording mark actually.